What is Diaphragm Valve?
Diaphragm valves (or membrane valves) consist of a valve body with two or more ports, an elastomeric diaphragm, and a “weir or saddle” or seat upon which the diaphragm closes the valve. The valve body may be constructed from plastic, metal, wood, or other materials depending on the intended use.
Diaphragm valves get their name from a flexible disc that comes into contact with a seat at the top of the valve body to form a seal. A diaphragm is a flexible, pressure-responsive element that transmits force to open, close, or control a valve.
Diaphragm valves are related to pinch valves, but use an elastomeric diaphragm, instead of an elastomeric liner in the valve body, to separate the flow stream from the closure element.
How do diaphragm valves work?
Diaphragm valves are simple in construction and operation. A valve actuator is in contact with the inner membrane (or “diaphragm”) of the valve in its open position. When the user desires the valve to be shut, the actuator is pressed and/or turned and the membrane is pushed into the edge of the solid damn, closing the valve.
Certain diaphragm valves can also have half open/restricted positions where the membrane is partially closed, allowing a throttled flow through the valve. In the next section, we will examine the broad categories of diaphragm valves and where they are most useful.
Method Of Control
Diaphragm valves use a flexible diaphragm connected to a compressor by a stud which is molded into the diaphragm. Instead of pinching the liner closed to provide shut-off, the diaphragm is pushed into contact with the bottom of the valve body to provide shut-off.
Manual diaphragm valves are ideal for flow control by offering a variable and precise opening for controlling pressure drop through the valve. The handwheel is turned until the desired amount of media is flowing through the system.
For start and stop applications, the handwheel is turned until the compressor either pushes the diaphragm against the bottom of the valve body to stop the flow or lifts off the bottom until the flow is able to pass through.
Diaphragm Valve Function
The diaphragm is connected to a compressor by a stud molded into the diaphragm. To start or increase flow the compressor is moved up by the valve stem. To stop or slow flow, the compressor is lowered and the diaphragm is pressed against the bottom of the valve.
Diaphragm valves are excellent for controlling the flow of fluids containing suspended solids and offer the flexibility of being installed in any position. Weir-type diaphragm valves are better at throttling than straight through diaphragm valves because the design’s large shutoff area along the seat gives it the characteristics of a quick-opening valve. The diaphragm acts as the gasket of the valve to seal against leaks between the body and bonnet cap.
Types of Diaphragm Valve
Diaphragm valves are separated into two major categories: diaphragm check valves and diaphragm control valves. Diaphragm check valves ensure materials flow unidirectionally by employing a flexible membrane diaphragm.
Primarily there are two basic designs of diaphragm valves: weir and straight-through types. The body interior and the end flanges can be lined to make the diaphragm valves suitable for corrosive applications. Various lining materials can be used, depending upon the application.
Weir-Type Diaphragm Valves
As shown in Fig. A, a weir is provided as an integral part of the valve body. The weir acts as the valve seat against which the diaphragm is compressed to stop the flow. This type of diaphragm valve is generally produced in large sizes. The raised weir reduces the amount of diaphragm travel from the fully open to the fully closed position, thus reducing the amount of stress and strain in the diaphragm.
Straight-Through Diaphragm Valves
Full-bore-type diaphragm valve illustrating the passage of ball-brush cleaner through the valve
When the straightway valve is open, its diaphragm lifts high for full streamline flow in either direction. When the valve is closed, the diaphragm seals tight for positive closure even with gritty or fibrous materials in the line.
The full-bore type of valve is most extensively used in the beverage industry. It permits ball-brush cleaning with either steam or caustic soda, without opening or removing the valve from the line.
Additional Diaphragm Valve Types
Other diaphragm valve types include process valves, zero static valves, sanitary diaphragm valves, diaphragm solenoid valves, direct-acting diaphragm solenoid valves, indirect-acting diaphragm solenoid valves, shut-off valves, pneumatic diaphragm valves, and diaphragm actuated gate valves.
- Process valves are any mechanical diaphragm designed to regulate, start, or stop a flow process.
- Zero static valves are another diaphragm valve type that are great for clean operations, because they eliminate the opportunities for bacterial growth and flow stagnation.
- Sanitary diaphragm valves help their users avoid product contamination. They are made from sterile materials that foster an aseptic atmosphere.
- Diaphragm solenoid valves are a variation on the regular solenoid valve, which is an electromechanical valve used for flow control. Along with a solenoid coil, diaphragm solenoid valves usually feature rubber diaphragms situated inside of the valve body. They open and close against a hard seat. They may be direct acting or indirect acting.
- Direct acting diaphragm solenoid valves change their diaphragm position only when the solenoid coil energizes. When this happens, the diaphragm opens or closes, depending if the valve is normally open or normally closed.
- Indirect acting diaphragm solenoid valves, also known as pilot operated diaphragm solenoid valves, require diaphragm pressure to work. They change their diaphragm position when the fluid flowing through a valve gathers enough pressure. In other words, the media pressure acts as a pilot, directing the diaphragm position.
- Shut-off valves are those valves capable of stopping, or shutting off, flow in a pipe using positive closure.
- Pneumatic diaphragm valves are automated diaphragm valves that use pneumatic controls with an elastomeric membrane. They are popular because they only require the compressed air to expand and contract the diaphragm materials.
- Diaphragm actuated gate valves are bi-directional flow safety valves. They combine the strategies used by diaphragm and gate valves to create an extremely reliable hybrid. They feature two floating seats, a slab gate, and a spring return diaphragm actuator.
Applications of Diaphragm Valves
- Clean or dirty water and air service applications
- De-mineralized water systems
- Corrosive applications
- Radwaste systems in nuclear facilities
- Vacuum service
- Food processing, pharmaceutical, and brewing systems
Advantages of Diaphragm Valve
- Can be used as on-off and throttling service valves.
- Offer good chemical resistance due to a variety of linings available.
- Stem leakage is eliminated.
- Provides bubble-tight service.
- Does not have pockets to trap solids, slurries, and other impurities. It is suitable for slurries and viscous fluids.
- These valves are particularly suitable for hazardous chemicals and radioactive fluids.
- These valves do not permit contamination of flow medium; thus, they are used extensively in food processing, pharmaceutical, brewing, and other applications which cannot tolerate any contamination.
Disadvantages of Diaphragm Valve
- The weir may prevent full drainage of piping.
- Working temperatures and pressures are limited by the diaphragm material. Generally, the pressures are limited to 200 psi (1380 kPa) and temperatures up to 400 F (204 C).
- The diaphragm may also limit the hydrostatic pressure.
- The diaphragm may experience erosion when used extensively in severe throttling service containing impurities.
- Diaphragm valves are available in limited sizes, usually NPS ¹⁄₂ to 12 (DN 15 to 300).